Malic acid is widely distributed in nature comprising up to almost 1% by weight of the content of apples. In mammals it is formed in the penultimate reaction in the Krebs cycle and after conversion to oxaloacetate. Malic acid is absorbed by the gut and has specific plasma membrane transporters. This protocol investigates whether provision of malic acid to patients with defects in the Krebs cycle will lead to clinical improvement. One infant has been treated at this time.